netdev - Re: [net-next 15/19] can: tcan4x5x: rework SPI access

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Message-ID: <fc1f14e9-9c20-f137-d131-67b4133a3074@pengutronix.de>
Date:   Tue, 12 Jan 2021 16:25:40 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        linux-can@...r.kernel.org, kernel@...gutronix.de,
        Dan Murphy <dmurphy@...com>, Sean Nyekjaer <sean@...nix.com>
Subject: Re: [net-next 15/19] can: tcan4x5x: rework SPI access

On 1/7/21 11:38 PM, Jakub Kicinski wrote:
> On Thu, 7 Jan 2021 22:17:15 +0100 Marc Kleine-Budde wrote:
>>> +struct __packed tcan4x5x_buf_cmd { 
>>> +	u8 cmd; 
>>> +	__be16 addr; 
>>> +	u8 len; 
>>> +};   
>>
>> This has to be packed, as I assume the compiler would add some space after the
>> "u8 cmd" to align the __be16 naturally.
> 
> Ack, packing this one makes sense.
> 
>>> +struct __packed tcan4x5x_map_buf { 
>>> +	struct tcan4x5x_buf_cmd cmd; 
>>> +	u8 data[256 * sizeof(u32)]; 
>>> +} ____cacheline_aligned;   
>>
>> Due to the packing of the struct tcan4x5x_buf_cmd it should have a length of 4
>> bytes. Without __packed, will the "u8 data" come directly after the cmd?
> 
> Yup, u8 with no alignment attribute will follow the previous
> field with no holes.

Ack, without __packed, there's no diff in the objdump on arm.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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